1. Field of the Invention
The present invention relates to a holographic recording device for recording interference fringes of 2 light beams on a holographic recording medium and a holographic recording method and, in particular, to a reduction of noise caused at a recording time of current interference fringes by preliminarily recorded interference fringes.
2. Background Art
A holographic data storage system for recording/reproducing a large amount of data by utilizing the holographic technology has been proposed recently. In such holographic data storage system, a multiple recording system is used in order to improve the recording density. In the holographic data storage system, a number of independent pages are recorded in one area. Typical examples of the multiple recording system are the angle-multiple recording system, the shift-multiple recording system and the phase-code multiple recording system and various other systems such as the speckle-multiple recording system are also known. When such multiple recording system is used, the recording capacity of the holographic data storage system becomes substantially large according to calculations. However, it is impossible in the existing circumstances to increase the capacity due to noise caused by previous interference fringe recording.
In recording data by using holographic technology, intensity of light on a holographic recording medium is large in the vicinity of an origin of the medium and light intensity on other areas is small as shown in FIG. 5. Incidentally, the high intensity component in the vicinity of the origin is called DC component. Such uneven distribution of light intensity is known as the DC component problem when a spatial light modulator (SLM), which performs only intensity-modulation is used. It is also known that, with such unevenness of light intensity distribution, various problems occur in a recording time and some countermeasures have been proposed in, for example, a publication “Holographic Data Storage; H. J. Coufal, D. Psaltis, G. T. Sincerbox ED; Springer; p. 259-269 Beam Conditioning Techniques for Holographic Recording Systems”.
One of the countermeasures is a method using a random phase mask or a phase-diffusion plate. In the method using the random phase mask as a phase diffusion plate, light is diffused by an optical element, which has, for example, a pitch identical to a pixel pitch of a SLM and a random pattern having phase-difference of 0 and π, correspondingly to the pixels of the SLM. This optical element having a random pattern having phase-difference of 0 and π is called the phase-diffusion plate. When light is diffused by the phase-diffusion plate, the light distribution on the recording medium becomes uniform and the recording characteristics are substantially improved. However, it is known that, although the problem of the DC component is solved by using the phase-diffusion plate, noise of an image is increased as described in page 264 of the above mentioned publication.
One of causes of noise will be described. As shown in FIG. 6A and FIG. 6B, in the angle-multiple recording, interference fringes formed by signal lights and a reference light are recorded in a same recording area of a holographic recording medium 50 while changing an incident angle of the reference light with respect to the recording medium. In FIG. 6A, interference fringes of the signal light 101 and the reference light 201 is recorded in the holographic recording medium 50, which is referred to as a first multiple recording, and, in FIG. 6B, interference fringes of the signal light 102 and the reference light 202 is recorded in the holographic recording medium 50, which is referred to as a second multiple recording. When the multiple recording is performed, both a recording signal and various noises are recorded. One of the causes of various noises will be described.
In FIG. 7, a reference numeral 4-1 shows interference fringes of the signal light 101 and the reference light 201 and a reference numeral 4-2 shows interference fringes of the signal light 102 and the reference light 202. These two interference fringes are indispensable primarily. On the other hand, the reason why unnecessary interference fringes are produced will be described with reference to FIG. 8. After the interference fringes 4-1 are recorded first on the holographic recording medium 50, the interference fringes 4-2 are recorded on the holographic recording medium 50 by irradiating the holographic recording medium 50 with the signal light 102 and the reference light 202. In this case, when irradiation with the signal light 102 is performed, the signal light 102 is diffracted by the interference fringes 4-1 recorded firstly on the holographic recording medium 50. When the signal light 101 and the signal light 102 are identical completely, light identical to the reference light 201 may be generated by this diffraction. The light identical to the reference light 201 thus generated is diffracted again by the interference fringes 4-1 recorded firstly, resulting in a light identical to the signal light 101. The light identical to the signal light 101 thus generated interferes with the reference light 202. As a result, unnecessary interference fringes 5-1 are recorded together with the interference fringes 4-2. Incidentally, although it is assumed in this description that the signal light 101 and the signal light 102 are completely identical, the light identical to the reference light 201 is practically generated with intensity proportional to mutual correlation of these signal lights.